Experimental Two-Photon, Three-Dimensional Entanglement for Quantum Communication

Orbital angular momentum entangled photons emitted by a down-conversion source are in higher dimensional entangled states. Here we report the experimental confirmation by demonstrating a violation of a generalized Clauser-Horne-Shimony-Holt–type Bell inequality in three dimensions by more than 18 standard deviations. Higher dimensional entangled states allow the realization of new types of quantum communication protocols. They also provide a more secure quantum cryptography scheme. Therefore our experimental results are likely to have applications in future quantum communication technology.

Figure 1

Principle of the experimental setup. The $UV$ beam at $\lambda =351\mathrm{n}\mathrm{m}$ pumps a BBO crystal set for type-I phase matching. This results in spontaneous production of an entangled photon pair. In the measurement stages, the angular momentum correlations are observed by having on each side detectors able to measure photons with $0$, $+\hslash$, and $-\hslash$ angular momentum, respectively. The corresponding outcomes are denoted as $0$, $1$, and $2$ for the lower detection unit and $0$, $2$, and $1$ for the upper detection unit. The coincidence logic identifies pairwise coincidences between the detectors on each side. The preparation stage allows one to prepare superposition of the eigenstates. This preparation stage generalizes the polarization rotator for linear polarization while the detector corresponds to a probabilistic polarization analyzer.

Figure 2

Two-photon, two-qutrit violations of a Bell-type inequality. The graph shows the frequency with which a violation of the generalized CHSH Bell inequality was observed. The upper limit for the correlation parameter for local realistic theories is 2.0. The inset shows the violations close to the theoretical quantum upper limit. The numbers should be compared with the total number of ${16}^4$ measured correlations altogether observed.